This invention relates, in general, to softening tissue product having fugitive wet strength properties; and more specifically, to a composition which may be applied to such tissue product for enhancing the softness thereof.
Sanitary paper tissue products are widely used. Such items are commercially offered in formats tailored for a variety of uses including facial tissues, toilet tissues and absorbent towels. Providing softness in such tissue and toweling products so as to allow comfortable cleaning without performance impairing sacrifices has long been the goal of the engineers and scientists who are devoted to research into improving tissue paper. Softness is a complex tactile impression evoked by a product when it is stroked against the skin. Softness has components arising from surface (fuzziness) properties as well as bulk (flexibility) properties.
There have been numerous attempts to improve the softness of tissue products. One area that has been exploited in this regard has been to engineer paper structures to take optimum advantages of the various available morphologies. U.S. Pat. No. 4,300,981, issued Nov. 17, 1981, discusses how fibers can be directed into various layers to be compliant to paper structures so that they have maximum softness delivery. While such techniques improve softness by the surface mechanism by generating more free fibers extending from the surface of the tissue, sometimes referred to as xe2x80x9cfree fiber endsxe2x80x9d or xe2x80x9cfuzz on edgexe2x80x9d properties, they unfortunately are often accompanied by a tendency of the tissue product to release fibers from its surface, a property referred to herein as xe2x80x9clintxe2x80x9d.
Another area receiving a considerable amount of attention is the addition of chemical softening agents, or xe2x80x9cchemical softeners,xe2x80x9d to tissue and toweling products. As used herein, the term xe2x80x9cchemical softenersxe2x80x9d refers to chemical ingredients that improve the softness of the tissue web to which they are applied. Chemical softeners impart a lubricious feel to tissue. As an example, they may include basic waxes such as paraffin and beeswax, and oils such as mineral oil and silicone oil as well as petrolatum, and more complex lubricants and emollients such as quaternary ammonium compounds with long alkyl chains, functional silicones, fatty acids, fatty alcohols and fatty esters.
Typically, chemical softeners are added in small amounts, less than about 5%, and generally less than 1%. One of the reasons for using such small amounts is a relatively high cost of the softeners compared to other papermaking ingredients. Another reason is that the chemical softeners, particularly when added prior to web drying or during web drying, may cause loss of paper strength and loss of adhesion of the paper sheet as it is creped from the Yankee dryer. Even if added after the web is formed, dried, (and creped if applicable) such as described, for example, in U.S. Pat. No. 6,162,329 the softeners may still cause some undesirable consequences, such as strength degradation and the capacity of the tissue web surface for holding such additives, and the specific degradation of the surface strength, which, in turn, raises the before-mentioned problem of xe2x80x9clint.xe2x80x9d
The benefits of adding fiber-flexibilizing compounds to a tissue paper have also been recognized in the art. The fiber-flexibilizing compounds can be classified into (a) xe2x80x9chumectants,xe2x80x9d defined herein as ingredients which raise the equilibrium moisture content in excess of that of the base paper to which they are applied, typically cellulosic in nature, and (b) xe2x80x9cplasticizers,xe2x80x9d defined herein as compounds which do not themselves result in an equilibration to a particularly high moisture content with humidity, nonetheless, their intrinsic molecular properties emulate the effect on fibers similar to that observed as the moisture content is raised. In other words, plasticizers have the effect of making fibers flaccid without substantially raising the moisture content of the fibers. Applicants recognize that a compound classified as a humectant can, and typically does, intrinsically possess plasticizer properties itself, i.e. capability to plasticize fibers beyond a level expected due to the moisture which it is able to attract and/or retain within the fiber.
Despite the recognized advantages of the fiber-flexibilizing compositions, their utility with respect to paper softening has been limited primarily because (a) the amount of the composition needs to be limited due to deposition methods requiring either wet-end addition or press-section addition, i.e. so-called xe2x80x9cwet-webxe2x80x9d addition methods; and (b) the need to provide a so-called fugitive wet strength in the products most desiring the flexibilizing treatment.
It is important for certain tissue papers, especially those which will be disposed through sewer and septic systems, to have fugitive wet strength. Fugitive wet strength is defined herein as the percentage of wet tensile strength reduction observed when measured about 30 minutes after wetting, compared to that measured immediately upon wetting. Fugitive wet strength is typically induced by providing ketone or aldehyde functional groups in the cellulose or in an additive to the cellulose fibers so that inter-fiber acid-catalyzed hemiacetal bond formation can occur. These chemical bonds are initially resistant to breakage even when wetted, but slowly lose such resistance to hydrolysis in water such that the wet tensile strength decays with time. Thus, the fugitive wet strength permits delivery of a product having a high initial wet tensile strength so that it can be used in a moist condition, while allowing for eventual breakdown in the septic or sewer systems.
The addition of both the fiber-flexibilizing compositions and fugitive wet strength to the same tissue structure are competing needs. The fiber-flexibilizing compositions typically require water-holding media or compounds that have chemical behavior characteristics of water or compounds that require large amounts of water to be transported to the paper structure. At the same time, the fugitive wet strength begins to decay in contact with such media suited for the fiber-flexibilizing compositions.
Accordingly, there is a need for materials for economically softening paper structures having fugitive wet strength without unduly degrading dry strength, including maintaining surface integrity, i.e. low lint.
The present invention is directed to a tissue product comprising cellulosic fibers and having at least about 10% of fugitive wet strength and at least about 3% of a fiber-flexibilizing composition. More specifically, the tissue product of the present invention can have at least about 25% of the fugitive wet strength. The fugitive wet strength can be generated by adding a binder that promotes acid-catalyzed formation of hemiacetal functional inter-fiber cross-links. The fiber-flexibilizing composition can comprise either a humectant or a plasticizer. The humectant can be selected from the group consisting of calcium chloride; lactic acid and its salts, high fructose corn syrup, glycerol, triacetin, sorbitol, maltitol, mannitol, propylene glycol, and any combination thereof. The plasticizer can be selected from the group consisting of urea, alkyloxylated glycols, dextrose, sucrose, ethylene carbonate, propylene carbonate, and any combination thereof.
The tissue product can comprise a single-ply structure or a multi-ply structure. The tissue product can comprise a foreshortened, for example creped, tissue. Alternatively, the product can be uncreped. The tissue web can be made by any process known in the art, including, without limitation, a conventional papermaking process and a through-air-drying papermaking process.
The tissue product can comprise a differential-density paper comprising a plurality of high-density micro-regions and a plurality of low-density micro-regions. In the latter instance, the tissue product""s plurality of high-density micro-regions can comprise a substantially continuous network, a substantially semi-continuous pattern, or a pattern of discrete areas, while the plurality of low-density micro-regions can comprise, correspondingly, a pattern of discrete fibrous pillows encompassed by the network region, a substantially semi-continuous pattern, or a substantially continuous areas.
A process for making the tissue product of the present invention comprises the steps of providing a plurality of cellulosic fibers comprising fugitive wet strength agent; forming a web of the cellulosic fibers; heating the web to a temperature of at least about 40xc2x0 C. and a moisture content of less than about 5%; and depositing a fiber-flexibilizing composition to the surface of the web.
The step of forming a web of the cellulosic fibers can comprise the steps of providing a forming belt; depositing the plurality of cellulosic fibers comprising fugitive wet strength agent onto the forming belt and forming an embryonic web of the cellulosic fibers on the forming belt; providing a fluid-permeable macroscopically monoplanar molding belt having a web-side, a backside, opposite to the web-side, and a plurality of deflection conduits extending from the web-side to the backside and structured to receive portions of the cellulosic fibers therein; transferring the embryonic web from the forming belt to the web-side of the molding belt; deflecting portions of the embryonic web into the deflection conduits of the molding belt; impressing the embryonic web against the web-side of the molding belt; and drying the embryonic web. The step of deflecting the fibers into the deflection conduits of the molding belt can comprise applying a fluid pressure differential to the plurality of fibers disposed on the molding belt.
The steps of impressing the embryonic web and drying the embryonic web can comprise pressing the embryonic web between the molding belt and a surface of a drying drum. The step of depositing a fiber-flexibilizing composition to the surface of the web can comprise spraying the fiber-flexibilizing composition onto the surface of the web, printing fiber-flexibilizing composition onto the surface of the web, extruding fiber-flexibilizing composition to the surface of the web, or any combination thereof.